As was mentioned, since you have not done darkroom work before, start out small. This way the cost of you learning how to enlarge is reduced. The bigger paper is EXPENSIVE, so you do not want to waste it learning how to print. When you can reliably print 8x10, then you can move up, one paper size at a time, because you also have to learn how to handle the bigger paper.

As David said, if you have the space, I would get a 4x5 enlarger. If you are space constrained or have to pack up or move the enlarger due to not having a permanent setup, then a MF enlarger 6x6 or 6x7 size. According to my D5 manual, a standard D5 chassis with an 80mm lens will make 10.2x enlargement, an XL chassis will make a 14.2x enlargement. So if you have the vertical height available, an XL chassis is a better long term choice...as long as you have the vertical height so it fits. My D5 manual says max height of D5XL is 64 inches. That is 5 feet 4 inches tall !!! To focus, you will need a small step ladder to get up that high.

You also need to plan how you will process the LARGE print.

For larger than 11x14, or even starting at 11x14, I would go with single tray or drum, to avoid handling a BIG sheet of wet paper.
Honeywell and Heath/Mitchell had rocking trays up to 16x20 that you can use for single tray processing. I used the Honeywell 11x14 rocking tray and it worked well. You have to search the used market for these rocking trays.
For LARGE size, same as PKM-25, I would use a drum. The trick is finding one that big. My biggest drum is 16x20.

Finally how will you wash the print? BIG washers are EXPENSIVE, a 20x24 will probably cost well over $500...used. The only low cost alternative that I know of is the Kodak washing tray syphon.